1. Field of the Invention
The present invention relates to a hologram recording and reproducing apparatus comprising a hologram record element including a record unit formed by an optical single crystal having an optically induced refractive effect and an electrode means provided on said record unit for applying a voltage to said record unit, and an optical system for impinging light onto an incident surface of said record unit.
2. Related Art Statement
A holography has been used in various applications such as interference measurement, optical information processing, optical element and three-dimensional display as a unique technique for reproducing a complete wave front. In order to perform hologram recording and hologram interference, a silver salt photosensitive material has been generally used. However, the silver salt photosensitive material requires a developing process, and thus it is impossible to realize a real time hologram. It has been proposed to use a single crystal having an optically induced refraction effect such as Bi.sub.12 SiO.sub.20 as a real time hologram record element. Such a single crystal is also called a photorefractive crystal. The photorefractive crystal has been disclosed in, for instance "Laser Science Study Report", 1990 March pp. 1-9 under a title of "Holography Recording Property of BSO Single Crystal". In this reference, object light and reference light are made incident upon a major surface having a crystal orientation (110) of a Bi.sub.12 SiO.sub.20 single crystal and an electric field of several kV/cm is applied in a direction perpendicular to an interference fringe by means of electrodes provided on both end faces of the (110) surface.
In applications of interference measurement, optical information processing and optical element, a size of hologram record element is limited by a purpose of respective applications, but it is not always necessary to use large hologram record elements. However, in an application of three-dimensional display, it is absolutely required to use a hologram record element having a size larger than a certain limit, because in order to attain a stereoscopic view by utilizing the parallax of a human being, a hologram record element should be not less than a pupil distance between right and left eyes. Usually, a size of the hologram record element should be larger than about 50 mm.
A size of the above mentioned real time hologram record element is physically limited by a size and shape of the Bi.sub.12 SiO.sub.20 single crystal. Usually the Bi.sub.12 SiO.sub.20 single crystal hologram record element has a size of ten and several mm.times.ten and several mm. Due to this limitation on the size, the application of the Bi.sub.12 SiO.sub.20 single crystal hologram record element has been practically restricted to the interference measurement and optical information processing. The real time holography has been earnestly desired to be developed in a near future as an output device of a three-dimensional image display system in a three-dimensional CAD system. Therefore, up-sizing of the real time hologram record element has been strongly desired.
The inventor has studied and conducted various experiments for realizing a large size hologram record element which may be used in the three-dimensional display, and has been encountered the following problem.
In the three-dimensional holography, a viewer stands in front of the hologram record element such that the left and right eyes are faced with the hologram record element perpendicularly thereto and reproducing light is made incident upon the hologram record element from a side opposite to the viewer. During this reproduction, when a room in which the hologram recording and reproducing apparatus is arranged is bright, undesired light emanating from surrounding objects is made incident upon the hologram record element surface and is reflected thereby toward the viewer. Then, the reproduced three-dimensional image monitored by the user might be disturbed greatly.
Now the above problem will be explained further in detail. FIGS. 1A and 1B are schematic views for explaining the problem encountered upon viewing the three-dimensional image by using the known hologram recording and reproducing apparatus. In the known apparatus, the hologram record element is formed by (110) wafer of the photorefractive single crystal such as Bi.sub.12 SiO.sub.20 single crystal. It should be noted that the Bi.sub.12 SiO.sub.20 single crystal belongs to the cubic system, and therefore &lt;100&gt; axis, &lt;010&gt; axis and &lt;001&gt; axis are equivalent. In the known apparatus, the hologram record element is formed by cutting a (110) wafer. As illustrated in FIG. 1A, an optical system 6 is installed within a black box 5 and a hologram record element 3 is provided on one side wall of the black box. Reproducing light 4 is made incident upon an incident (110) surface of the hologram record element 3 in a direction substantially perpendicular thereto. A viewer 1 is standing at a point opposite to the optical system 6 with respect to the hologram record element 3. A viewing direction 2 of the viewer 1 is substantially perpendicular to an exit (110) surface of the hologram record element 3, which is opposed to the incident (110) surface of the hologram record element. It should be noted that the viewing direction 2 is opposite to the incident direction of the reproducing light 4 emitted by the optical system 6.
The optical system 6 is provided within the black box 5, so that any undesired light is not made incident upon the hologram record element 3 from the direction of the reproducing light 4. As shown in FIG. 1A, the exit (110) surface of the hologram record element 3 has to be exposed out of the black box 5 such that the viewer 1 can see the reproduced three-dimensional image.
The object light for recording interference fringes in the hologram record element 3 and reproducing light 4 are emitted from laser light sources. However, an amount of light emitted by the laser light source is limited. Particularly, in order not to increase a size of the whole hologram recording and reproducing apparatus, it is necessary to use as small light sources as possible. Therefore, the brightness of the reproduced image could not be made sufficiently high.
When the hologram recording and reproducing apparatus is used to display a three-dimensional image, it is required to see the three-dimensional image by the viewer 1 under a bright lighting condition including a room lighting device 8 as illustrated in FIG. 1B. However, in such case, light 9 directly emitted by the lighting device 8 and light rays reflected by surrounding substances might be made incident upon the exposed (110) surface of the hologram record element 3. Then, the incident light 9 is reflected by the hologram record element 3 and reflected light 10 is made incident upon the viewer 1. At the same time, the reproduced image light is made incident upon the viewer 1 as depicted by an arrow 7. In this manner, the undesired light 10 reflected by the hologram record element 3 is superimposed on the reproduced image light 7. As stated above, the reproducing laser light has a limited intensity and the intensity of the room lighting device 8 is relatively larger than the reproducing light, the reproduced image could not be observed under a good condition.
In order to eliminate the undesired reflection by the hologram record element 3, the inventor has designed a hologram record element shown in FIG. 2. In this hologram record element 11, a record unit 12 is formed by a rectangular flat plate made of the photorefractive crystal, a pair of electrodes 13 are provided on opposite end faces of the record unit, the electrodes being connected to a DC voltage supply source 15 by means of conductors 16. A anti-reflection film 14 is applied on a major surface 12a of the record unit 12 which is to be faced by a viewer.
The anti-reflection film 14 is formed by a dielectric film which has a specific resistance of at most about 10.sup.9 .OMEGA..multidot.cm. The record unit 12 has a very high specific resistance such as on the order of 10.sup.14 .OMEGA..multidot.cm. Therefore, the specific resistance of the anti-reflection film 14 is relatively lower than that of the record unit 12. During the recording and reproducing, it is necessary to apply a very high voltage to the hologram record element 11. When the anti-reflection film 14 having a relatively low specific resistance is applied on the surface of the record unit 12 and a high voltage is applied across the electrodes 13, there might occur undesired breakdown. Therefore, it is impossible to apply a high voltage without causing the breakdown.
Furthermore, in order to mitigate the undesired reflection by the hologram record element, the inventor has designed a hologram recording and reproducing apparatus, in which an hologram record element is inclined with respect to a reproducing light flux. As a result, it was found that a diffraction efficiency might be greatly reduced due to a refraction of an object light flux.